1. Field of the Invention
The present invention relates generally to polypeptides containing structural motifs associated with cell signaling and other biologically relevant activities, compositions comprising such polypeptides, and methods of identifying and using same.
2. Description of the Related Art
Aminoacyl-tRNA synthetase (AARS) proteins, which catalyze the aminoacylation of tRNA molecules, are essential for decoding genetic information during the process of translation. Each of the eukaryotic tRNA synthetases consists of a core enzyme, which is closely related to the prokaryotic tRNA synthetase, as well as additional domains that are appended to the amino-terminal end, carboxyl-terminal end or inserted into a region internal to the core enzyme. Human tyrosyl-tRNA synthetase (TyrRS), for example, has a carboxyl-terminal domain that is not part of prokaryotic and lower eukaryotic TyrRS molecules. Several aminoacyl-tRNA synthetases have been demonstrated to have non-canonical functions distinct from their involvement in translation. For example, mini-tyrosyl tRNA synthetase (mini-TyrRS), the N-terminal domain of TyrRS which corresponds to amino acid residues 1-364 and is cleaved by polymorphonuclear cell elastase and plasmin, exhibits non-canonical activities not found in the full-length protein. In vitro, mini-TyrRS has been shown to stimulate neutrophil activation and chemotaxis, endothelial cell proliferation and migration, and is pro-angiogenic in chick chorioallantoic membrane (CAM) and mouse matrigel assays. Mini-TyrRS has an ELR motif that, like CXC-chemokines such as IL-8, is involved in many of its chemokine and angiogenic activities. As in other ELR-containing cytokines, mutation of this motif inhibits mini-TyrRS binding to and stimulation of leukocytes and angiogenesis.
In addition, truncated forms of TrpRS have been demonstrated to have anti-angiogenic properties. In normal human cells, there are two forms of TrpRS that can be detected: a major form consisting of the full-length molecule (amino acid residues 1-471) and a minor truncated form. The minor form is generated by the deletion of an amino-terminal domain through alternative splicing of the pre-mRNA and is termed mini-TrpRS. The amino-terminus of miniTrpRS has been determined to be the methionine residue at position 48 of the full-length TrpRS molecule. Alternatively, truncated TrpRS can be generated by proteolysis. For example, bovine TrpRS is highly expressed in the pancreas and is secreted into the pancreatic juice, thus resulting in the production of a truncated TrpRS molecule. Additional studies indicate that miniTrpRS inhibits VEGF-induced cell proliferation and migration (Wakasugi et al., Proc. Natl. Acad. Sci. 99: 173-177 (2002)). In particular, a chick CAM assay shows that miniTrpRS blocks angiogenic activity of VEGF. In contrast, the full-length TrpRS does not inhibit angiogenesis. Thus, removal of the first 48 amino acid residues exposes the anti-angiogenic activity of TrpRS. Therefore, as with TyrRS, certain forms of TrpRS possess activities other than the aminoacylation of tRNA.
Other cytoplasmic proteins have also been shown to have isoforms with extracellular biological activities. For example, human thioredoxin (Hu-Trx) serves a key role in redox activities within the cytoplasm of cells, but has a truncated secreted form that is a mitogenic cytokine (Arner, 2000; Eur J biochem 267:6102-6109).
Unfortunately, there are no reliable methods available for predicting or identifying which cytoplasmic proteins or polypeptide fragments derived from AARS proteins and/or other proteins may have novel and previously unappreciated activities. The present invention addresses these needs and offers other related advantages.